FIG. 1 illustrates a conventional data acquisition circuit 10 for use in imaging systems, for example, computer tomography (CT) systems, to count energy quanta received from a target being imaged. Such circuits 10 typically include a detector 12 generating current charge pulses responsive to received energy 11, such as x-ray photons incident on the detector 12. The output from such a system may include pulse counts 64. An amplifier 14, such as an integrating trans-impedance amplifier, typically comprising an operational amplifier (op amp) 15 with an integrating capacitor 16, resistor element 18, and low impedance switch 119 connected in feedback between an inverting terminal and an output terminal of the op amp 15, converts the current charge pulses into voltage pulses. Optionally, parallel integration processing of the amplifier output 19 via a sample and hold circuit 74 and an analog to digital converter 72 may be used to provide a digital integration output signal 70 indicative of the power of radiation 11 received at the detector 12.
Resistor 18 and switch 119 may not necessarily be discrete devices, but may be implemented using one or more transistors, such as a Field Effect Transistors (FETs) where the resistance of such a device may be controllable by an applied voltage. Alternately, the switch 119 may be light activated such that it is closed by application of light pulse incident on the switch. The resistive element 18 provides for the reset of charge built up across the capacitor 16 as the op amp 15 responds to the charge pulses from detector 12.
A continuous reset circuit is a type of circuit having a constant resistance value for resistor element 18. A triggered reset circuit is a type of circuit that implements the low impedance switch 119 in the feedback circuit. Most of the time the low impedance switch 119 is configured in the open state and the charge from the detector 12 is integrated on the capacitor 16 with constant reset from resistance 18. The switch 119, for example, under control of a threshold circuit or periodic clock, closes for a short period in order to discharge an accumulated charge on capacitor 16 and prevent saturation of op amp 15. During the period of time when the switch 119 is closed, there is no response to charge pulses generated in detector 12. Therefore, it is desirable to minimize the period when the switch 119 is closed in order to detect all the X-rays incident to the detector. An example of threshold controlled circuit is a type of circuit that tests the direct current level of the op amp output 19 against a threshold and closes the switch 119 for a fixed number of clock cycles in order to discharge the capacitor 16.
The voltage pulses generated by the amplifier 14 are further processed by a pulse shaper 20 to generate a shaped voltage pulse that is provided to a discriminator 22. The discriminator 22 increments a count register 24 when the amplitude of the shaped voltage pulse exceeds a discrimination threshold. Accordingly, the data acquisition circuit 10 is configured for counting current charge pulses generated by the detector 12 that are representative of photons incident on the detector 12. Multiple counters 24 and correspondingly multiple discriminator thresholds may be used to bin events with respect to the shaped pulse amplitude. Multiple discrimination and counting into multiple bins is a coarse method for recording the energy of the photons incident on the detector 12. In some applications, the detector 12 may comprise one or more pixels, wherein one or more pixels are connected to a respective data acquisition circuit 10 via a binning switch 40.
In imaging systems such as CT systems, it is desired to achieve a relatively high count capability with sufficient energy resolution. In particular, avoidance of saturation of the amplifier at high count rates requires that a relatively low resistance resister 18 be used in the circuit 10 or that the switch 119 be reset often. However, a low resistance resistor 18 may result in decreased energy resolution due in part to the known phenomenon of ballistic deficit. Ballistic deficit results because the resistor 18 discharges the capacitor 16 during the period in which the signal charge pulse generated in detector 12 is being received at the input 17 of the op amp 15. An amplitude of a charge integrated on capacitor 16 is thereby decreased and, with reference to a noise level, there is a decrease in energy resolution of the circuit. Furthermore, the amplitude may be variable due to differences in the time duration of different charge pulses created in the detector 12. Variability in a charge pulse amplitude results in a decreased energy resolution of the circuit.
Energy resolution may also be affected by a shaping time of the pulse shaper 20. The longer the shaping time used by the pulse shaper 20, the better the higher frequency noise can be rejected, resulting in lower noise in a shaped voltage pulse. However, if the shaping time is made too long, this may result in excessive circuit dead time that can lead to overlap of shaped pulses and a distortion in the amplitude of the shaped pulse. In such a case there may be miscounting in the discriminator 22 and error in an energy assignment. Consequently, as a count rate increases, each shaped voltage pulse may incorporate signals from multiple detection events in an effect known as pile up. Accordingly, it is desired to improve the performance of an imaging data acquisition circuit over a wide range of energy input conditions.